This invention relates to scales for measuring the weight of an object, and more particularly to transducers for converting the weight of an object into an electrical signal property indicative of the weight of the object.
Transducers including flexure mode piezoelectric crystal resonators have been developed which are capable of converting a force applied to the crystal resonator into an electrical signal property representative of the applied force. One such resonator is shown in EerNisse U.S. Pat. No. 4,215,570 entitled "Miniature Quartz Resonator Force Transducer". "Flexure mode" refers to the particular mode of oscillation of the resonator. This mode is characterized by oscillatory flexural bending of relatively long, slender crystal members. Resonators having this mode of oscillation are typically relatively delicate structures which are prone to break is subjected to mechanical shock or to loading which applies any significant bending, torsion, or shear force to the resonator member. The resonator is strongest and most accurate when loaded as a column in pure tension or compression.
The flexure mode crystal resonator need not be piezoelectric, but may alternatively be a non-piezoelectric crystal structure (e.g., a silicon crystal) driven in the flexure mode by piezoelectric material (e.g., zinc oxide) associated with the crystal. Although in the preferred embodiment of the present invention the resonator is a piezoelectric crystal, it will be understood that the term "flexure mode crystal resonator" as used herein and in the appended claims includes both flexure mode piezoelectric crystal resonators and piezoelectrically driven flexure mode crystal resonators.
A problem which is relatively unique to weighing scales is that the line of action of the force due to gravity of the object to be weighed cannot be predicted with certainty. Unlike devices such as pressure transducers and accelerometers in which the line of action of the force to be measured is always the same, the line of action of the force in a weighing scale depends on where the user places the object to be weighed on the load platform of the scale. It is of the utmost importance to the proper and satisfactory operation of a weighing scale that the indicated weight of the object not depend on its location on the load platform. In addition, in a scale employing a flexure mode crystal resonator of the type mentioned above, the resonator must be protected from the bending, torsion, or shear force components which typically result from off-center loading of the scale. These force components not only increase the risk of resonator breakage, they also subject the resonator to load components which reduce the accuracy of the transducer.
Others have developed weighing scale transducers including thickness shear mode (as distinguished from flexure mode) piezoelectric crystal resonators. The crystal resonators in those transducers were relatively large and robust quartz crystal discs about 1.5 cm in diameter (see, for example, Walker U.S. Pat. No. 4,130,624). These transducers included (1) two vertically spaced, substantially parallel flexure assemblies for supporting the load platform of the scale, (2) a thickness shear mode crystal resonator disc mounted for loading by a compression force applied at two diametrically opposite points on the edge of the disc, and (3) a coupling assembly for applying vertical force from the flexure assemblies to the edge of the resonator disc. The coupling assembly included pivotal bearings for preventing application of moments from the flexure assemblies to the resonator disc. The coupling assembly also included, between the pivotal bearings and the resonator disc, a second system of vertically spaced, substantially parallel flexure assemblies for insuring vertical alignment of the coupling assembly. In addition to the mechanical differences between those transducers and the transducers of the present invention, there was no suggestion in that work of employing much more fragile flexure mode crystal resonators or even of a structure capable of mounting such a flexure mode resonator.
In view of the foregoing, it is an object of this invention to provide a weighing scale transducer employing a flexure mode crystal resonator.
It is a more particular object of this invention to provide structures for assuring that the flexure mode crystal resonator element in a weighing scale of the type mentioned above is subject only to axial loading and is prevented from receiving any significant bending, torsion, or shear loading regardless of the location of the object to be weighed on the load platform of the scale.